1. Field of the Invention
The invention provides a method for changing sound wave frequency, particularly provides a method for changing the wave frequency of an ultrasonic transducer by using the acoustic matching layer.
2. Description of the Prior Art
The ultrasonic transducer exhibits its characteristics without destroying the target material's structure (e.g., the human cells) , thus it is generally applied to the sensing, measuring and medical applications. The wave generation of the ultrasonic transducer is typically provided by the ferroelectric ceramic or composite materials; which have much higher acoustic impedances than that of water or air; there will be a large amount of energy loss at the interface between the ferroelectric material and the transduction medium. Thus, an acoustic matching layer is required to reduce such a large impedance mismatch, in order to prevent great energy loss at the interface between the transducer and the measured matter, and to improve the efficiency of ultrasonic transmission.
At present, polymer and polymer-based composite materials are widely adopted to produce the passive-type acoustic matching layers. The matching layer with an acoustic impedance value between the acoustic impedance values of the ultrasonic transducer and the transduction medium can be designed to lower the mismatch of acoustic impedances at the interfaces.
At present, most acoustic matching layers are made of polymer and polymer-based composite materials. The acoustic impedance (Z) of the matching layer can be adjusted by varying the mixing ratio of the ceramic/metal powders and polymer, achieving a value of the following:Zacoustic matching layer={square root over (Ztransducer×Ztransduction medium)}.
In addition, the ceramic/metal-polymer composite materials can be easily processed, and precisely cut to the required thickness (i.e. a quarter of the wavelength of ultrasound wave in the matching layer material). Thus, the above-mentioned passive-type acoustic matching layer design has been widely adopted in the transducer industry.
As shown in U.S. Pat. No. 6,989,625, the acoustic matching layer is made of silicon dioxide gel, and the thickness of the acoustic matching layer is equal to the quarter of the wavelength of ultrasound wave travelling in this material. As shown in another U.S. Pat. No. 6,969,943, the acoustic matching layer is made of the mixture of polymer and silicon dioxide, or aluminum oxide gel, and the thickness of the acoustic matching layer is equal to the quarter of the wavelength of ultrasound wave in this material. As shown in another U.S. Pat. No. 5,418,759, the acoustic matching layer is made of the mixture of copper powder and epoxy, and the thickness of the acoustic matching layer is equal to the quarter of the wavelength of ultrasound wave in this material.
However, the existing acoustic matching layers are not capable of filtering and adjusting the output frequency of the acoustic component actively. The output frequency of a commercial ultrasonic probe is typically kept at a constant. If two different output frequencies are required, two ultrasonic probes must be adopted and their focuses are overlapped at the same spot. However, the acoustic confocal procedure is difficult to achieve precisely, making it undesirable in many applications.